Scuff sheet separators typically include a feed means, for example, a belt or a roller, having a high coefficient of friction, which frictionally engages and separates the outside sheet from a stack. To prevent double feeds a retard means is placed opposite and forms a nip with the feed means. Typically the retard means has a coefficient of friction intermediate that of the feed means and the sheets. It may be stationary or be rotatably driven opposite to the feed means. The retard means retains any second sheet drawn into the nip because the outside sheet preferentially slides on the retarded second sheet. When only one sheet is in the nip the higher coefficient of friction of the feed means dominates that of the retard means and the sheet is fed. To accommodate sheets of different thickness the retard means is commonly mounted to be pivoted or otherwise moved away from the feed means by the sheets. See, for example, U.S. Pat. Nos. 2,273,287; 3,831,928; 3,984,095 and 4,544,147. Unfortunately, the competing frictions on the single or last sheet have a tendency to damage it if the retarding friction is high enough to reliably separate sheets.
U.S. Pat. Nos. 2,665,906 and 4,480,827 disclose top scuff separating devices in which the problems of slippage when feeding the final sheet have been reduced by use of a braked retard roller opposite a larger drive roller. The brake force on the retard roller is such that the retard roller rolls only when subjected to the increased tangential force of a single sheet or no sheet being in the nip.